Novel precursors polymerized from (alkylamino)borazines (AAB) were synthesized and transformation of processable poly-AAB to boron nitride (BN) was researched. The AAB monomers of the type (BNH)3(NHR)3 were synthesized via ammolysis of 2,4,6-trichloroborazine (TCB) with different propylamines under mild conditions. The specially designed monomers served as molecular precursors for BN by the polymer-derived ceramics route. The processability of the polymeric precursors varied with propylamino-groups of AAB linked with boron atoms on (BNH)3. The good processability of the poly[2,4,6-tris(iso-propylamino)borazine] (PTPiAB) was proven by melt-spinning it into polymer fiber. Furthermore, the PTPiAB gave a ceramic yield of about 53wt% in Ar at 1200°C by TGA. Based on FTIR, Raman, XRD, XPS and elemental analysis, the pyrolytic product of PTPiAB showed a composition of BN1.07. In addition, the BN illustrated excellent oxygen resistance in air.